2.25: Boyle's Law

Are you one of those people who can’t resist popping the bubbles of bubble wrap? As annoying as it might be to the people around you, do you just love to squeeze those little plastic bubbles? Before the bubbles pop, the air pressing against the inside of the bubbles inflates them like tiny balloons. When you pop the bubbles, most of the air rushes out.

Pop Goes the Bubble!

What does popping bubble wrap have to do with science? Actually, it demonstrates an important scientific law, called Boyle’s law. Like other laws in science, this law describes what always happens under certain conditions. Boyle’s law is one of three well-known gas laws, which state the relationships among temperature, volume, and pressure of gases. (The other two gas laws are Charles’ law and Amontons’ law.) According to
Boyle’s law
, if the temperature of a gas is held constant, then decreasing the volume of the gas increases its pressure—and vice versa. That’s what happens when you squeeze the bubbles of bubble wrap. You decrease the bubbles' volume, so the air pressure inside the bubbles increases until they pop.

”Father” Boyle

Boyle’s law is named for Robert Boyle, the English scientist who discovered this relationship between gas volume and pressure. Boyle based the law on his own controlled experiments. He published his results, along with detailed descriptions of his procedures and observations, in the 1660s. These steps were unheard of in his day. Mainly because of his careful research and the details he provided about it, Boyle has been called the “father of modern chemistry.”

It’s Crowded in Here

Imagine a container of gas molecules like the one in the
Figure
below
. The container in the sketch has a lid that can be pushed down to shrink the volume of the gas inside. Notice what happens as the lid is lowered. The gas molecules crowd closer together because there is less space for them to occupy and they have nowhere else to go. Gas molecules have a lot of energy. They are always moving and bouncing off each other and anything else in their path. When gas molecules bump into things, it creates pressure. Pressure is greater when gas molecules occupy a smaller space, because the greater crowding results in more collisions. In other words, decreasing the volume of a gas increases its pressure. If you go to the URL below, you can watch a simulation of this.

You Go Up and I’ll Go Down

As the volume of gas in the container pictured above gets smaller, the pressure of the gas molecules becomes greater. When two variables change in opposite directions like this, the variables have an inverse, or “upside-down,” relationship.

Q:
How could you show an inverse relationship with a graph? Sketch a graph to show what the relationship between gas volume and pressure might look like. Let the x-axis represent volume (V) and the y-axis represent pressure (P).

A:
Did you sketch a graph like the one in the
Figure
below
? Let’s see why this graph is correct. Find the point on the line where volume is smallest. That’s were pressure is highest. Then find the point where volume is largest. That’s where pressure is lowest. Whenever you see a graph with this shape, it usually represents variables that have an inverse relationship, like gas volume and pressure.

Summary

Boyle’s law states that if the temperature of a gas is held constant, then decreasing the volume of the gas increases its pressure.

When gas molecules bump into things, it creates pressure. Pressure is greater when gas molecules occupy a smaller space, because greater crowding results in more collisions. This explains why decreasing the volume of a gas increases its pressure.

Vocabulary

Boyle’s law
: Gas law stating that, if the temperature of a gas is held constant, increasing the volume of the gas decreases its pressure.

Practice

You can see for yourself that Boyle’s law really works. At the following URL, simulate measuring the volume and pressure of two different gases. Use the graphing tool to make graphs of your data. Then answer the questions at the end of the activity to see how well you understand Boyle’s law.

Review

State Boyle’s law.

Explain why the pressure of a gas increases when its volume decreases.

Imagine blowing up a latex balloon like the one shown in the
Figure
below
. Assume that you tie the balloon to prevent air from escaping and then squeeze the balloon with one hand, so there is less space inside it. Then assume you use both hands (and maybe a friend’s hands) to squeeze the balloon so there is even less space inside. What do you think will happen to the balloon if you keep squeezing it to a smaller and smaller volume? Explain your prediction.